In the paper, it is elaborated the gas seepage model of goaf was established with the technical conditions for actual mining of fully mechanized top-coal caving face of a mine as well as the widths of oxidized zone of goaf with different horizontal distances and vertical distances of high drainage tunnel were analyzed with the Fluent numerical simulation software combining with the actual layout requirements of high drainage tunnel aiming at the autoignition of goaf caused by gas extraction in high drainage tunnel. The results showed that when the vertical distance of high drainage tunnel was 35 m and its horizontal distance was 25 m, the width of the oxidized zone of goaf was the smallest. The determination for reasonable position of high drainage tunnel is with the practical guiding significances in improving the gas extraction efficiency, reducing danger of autoignition of coal and ensuring green, safe and efficient fully mechanized top-coal caving face.